netifd: update vlanconf for tvid

Option to support vlan translation added to network uci file and
hence netifd updated. In case its a broadcom chip, that is, if
vlanconf is detected, then use this option to configure vlan
translation.
Test via autotest and also manually (to verify the p-bit is not
overwritten). Rest of the scenarios work as expected wih the
existing conf so no regression